Thanks go out to Charles and Cindy Black with Hidden Valley Hibiscus for publishing this interesting article on plant immune systems in their Volume 12 Issue 2 February 2011 newsletter.

Do Plants Have an Immune System?

Yes. That much we do know! Their immune system is commonly considered to be passive or innate. This means that unlike animals, plant immune systems don't send specialized immune cells through a bloodstream to all parts of the plant where they can aggressively recognize and fight off specific invaders. But honestly, we still know very, very little about plant immune systems. Every few years a new study comes out with amazing news that plant immune systems do wild and weird things that we never thought they could do. So the best we can say at this point is that we still have a LOT to learn about plant immune systems. We may well find a whole world of new information about immunity once we can truly understand how plants fight disease.

What Do we Know about Plant Immune Systems?

We know how to eliminate external pests and bugs, and we're pretty good at fighting many kinds of fungal infections. We've written many, many articles on these topics and have many pages of our website devoted to them. But what about plants that are infected with bacteria and viruses? Many gardeners and most farmers won't even try to cure plants with viral or bacterial infections. Modern science recommends that plants with these kinds of infections be thrown in the garbage as soon as the illness is discovered. That's all well and good for some of our more common garden variety plants! But what about our cherished hibiscus and rarer plants? For those of us more tender-hearted gardeners who get deeply attached to our plants, isn't there some way to try to cure a sick plant?

We're sad to say that we don't yet have the answers to that question. We're still working on it ourselves! But we thought our readers might like to hear some of the lines of inquiry that we're looking into, some of the newest research, and some ideas of things we are trying out on our own plants.

A Healthy Plant + Good Hygiene = Better Immunity

The best defense against any invading microbe, as you've heard many times before, is a healthy plant. Healthy plants have strong cell walls that keep out almost all diseases very effectively. The problem comes when a cell wall is broken into by some contaminated outside entity, such as biting insects carrying disease, or dirty human pruning shears. Those are probably the two most common ways that plant diseases are spread from one plant to another. Once a single cell wall is breached and infection is planted in that one cell, the disease can begin in the plant. A healthy plant even then will be able to wall off that one cell, flood it with protective chemicals, and prevent the disease from spreading. But if the insects or pruning shears cut through cell walls all over the plant, and many cells become infected, the plant has much less ability to fight off the infection.

We've all seen this happen. Our hibiscus plant gets a case of spider mites that we don't catch and cure immediately. The plant loses a lot of leaves, and even though we get rid of every last bug, the hibiscus just continues to go downhill. It grows baby leaves, but they look weird, turn brownish or yellowish or look deformed, and they fall off before they get to normal size. The plant keeps trying to grow new baby leaves everywhere, but they all go through this slow death. The plant may linger for months, or sometimes even years, in this state, and then finally some last straw kills it - a cold snap, a hot spell, too much or too little water, almost any stressor. This is a classic case of insects carrying infection into plant cells and infecting it so badly that the plant cannot recover. It's one of the saddest things we see in hibiscus, because the plant stays in that sickly state for so long, and yet we still can't save it. Or can we?

What Might Work to Help a Plant Fight Disease?

But maybe we can start saving these plants! All hope is not lost. Here are some ideas suggested by new science research into plant immune systems. At HVH we are trying as many of them as we can, but we welcome any and all information from other hibiscus lovers on what has worked on not worked for your hibiscus.

A Growth Enhancer ~ The First line of Defense for Sick Plants

The first line of defense is to pump your plant full of as many health-supporting nutrients as possible. Remember, good health alone will help your plant fight off disease all by itself. So as quickly as you spot any signs of stress, pump up your plant's nutrition, and fill it full of the hormones and building blocks that will help it pull itself through the disease. A good growth enhancer product will do this very quickly. We have saved more sick hibiscus with this product alone than everything else we have tried all together. If there is only one product you keep on your shelf for emergencies, it should be a growth enhancer.

Vitamins ~ The Research Cutting Edge

Most vitamins and many substances that we use for medicines are derived from plants where they grow naturally. Although we harvest these substances, analyze them, and recreate them for human and animal health purposes, we rarely stop to think about what purpose these substances serve within the plants they naturally occur in. The newest research is suggesting and confirming the suggestions that these substances serve healing purposes for the plants that produce them. There is not very much research yet in this area, but what there is is beginning to show promise. Here is a quick look at some of the findings.

Vitamin D

Several studies have found that Vitamin D watered into the soil promotes both increased root growth and increased plant growth in plants. One study thought this was possibly due to vitamin D's effect on increasing calcium absorption in plants, just as it does in animals. But with or without added calcium, vitamin D has had a positive effect on boosting the growth and healing of stressed plants.

Vitamin C

There have been mixed results with Vitamin C. Some plants have shown increased growth and ability to fight off bacterial disease with Vitamin C added to the soil or water. Other studies showed no improvement. All plants make their own vitamin C, so scientists hypothesize that if a plant has been able to make enough vitamin C on its own, extra vitamin C won't help it. But if it has been too sick to make enough of its own, the extra vitamin C will help. Until recently we didn't even know for sure if all the vitamin C plants make actually did anything useful for them. But a 2009 study from Cornell University proved that one plant species at least can't survive without vitamin C, that it was essential for photosynthesis, and that vitamin C was highly protective against several types of stressors, including air pollution, ozone, and ultraviolet radiation. The plants that didn't have any vitamin C grew "shriveled leaves." Other studies of other plant species have also shown that vitamin C is essential for growth and has a protective effect on plants that are under different kinds of stress, such as drought.

B Vitamins

B vitamins have been known to help plants resist disease for many years. More recent research confirms those earlier findings. One treatment of B vitamins has been shown to increase resistance to bacterial, viral, and fungal diseases for more than 2 weeks. All of the B vitamins researched so far seem to have a similar effect in studies on many different types of plants.

Aspirin

Amazingly, aspirin seems to be involved in plant immune activities at almost every level in almost every plant studied. It seems to be the near-universal plant cure-all, just as it was the near-universal human cure-all for centuries of medical history. But in plants aspirin does more than just alleviate pain. It actually blocks microbes and fights off bacterial, viral, and fungal disease. This immunity comes at a cost though. But it weakens a plant to have to make its own aspirin, and it makes the plant more vulnerable to insect attacks. This is one of the reasons that sick plants often seem to get better, then go downhill and get sicker than ever. The act of fighting off disease is very hard on plants, and the damage that follows the immune reaction may be worse than the original disease. This is why extra nutrition of every kind is so important for sick plants! They need all the help they can get when disease strikes.

No matter what you try, remember, the number one rule for sick or stressed plants is always WARMTH. Keep your plants as warm and toasty as you can, even if you need to reduce their light!

University of Chicago Medical Center, 2009. "Researchers Unravel Role Of Priming In Plant Immunity." ScienceDaily, http://www.sciencedaily.com/releases/2009/04/090402143449.htm.

Gassman, W., 2009. "Too Much of a Good Thing: Understanding Plants' Overactive Immune System Will Help Researchers Build Better Crops." University of Missouri, http://munews.missouri.edu/news-releases/2009/0527-Gassman-plant-defense.php.